This invention relates to a multi-fuel, combined cycle power plant.
For many years, large scale gas turbine based stationary power plants fueled by conventional gas or liquid fuels, such as natural gas, liquefied petroleum gas (LPG), etc., have been used by utilities for peaking purposes because of the fast, on-line response of a gas turbine.
More recently, combined cycle power plants have been constructed to increase the efficiency of the system thus enabling the gas turbine to be incorporated into power plants that provide base load electric power. Such power plants include a waste heat boiler that extracts heat from the exhaust gases of the gas turbine for generating steam that drives a steam turbine and produces additional power. Furthermore, sometimes supplementary duct firing has been used as an addition to the waste heat boiler.
At the same time, efforts have been made to utilize less expensive, and more abundant low heat content solid fuels, such as low quality coal, oil shale, bituminous phosphates, biomass, etc., in base-line power plants. Conventionally, power plants using such low heat content fuels have employed fluidized bed combustors that include integral heat transfer elements. Such elements are usually rapidly fouled by agglomeration of ash because the solid fuels used contain relatively large amounts of ash, and this complicates heat transfer to the working fluid of the power plant.
It is an object of the present invention to provide a combined cycle power plant operating on low quality solid fuel in which several of the disadvantages outlined are reduced or substantially overcome.
A power plant according to the present invention includes a gas turbine unit having a compressor for compressing ambient air, a burner for burning fuel under excess air conditions and heating air compressed by the compressor, a generator, and a turbine coupled to a generator for expanding air heated by the burner to drive the compressor and the generator for producing hot exhaust gases and electrical power. The plant further includes a combustor, for supplementary firing of the exhaust gases burning particles of solid fuel which are fluidized by the exhaust gases from the turbine and which produce hot products of combustion that include coarse ash particulate. Apparatus is provided for generating power from the hot products of combustion.
Preferably, the fluidization of the particles in the combustor is achieved in a spouted bed combustor that includes a separator for extracting coarse ash particulate from the hot products of combustion. In such case, the apparatus for generating power from the hot products of combustion includes a waste heat boiler responsive to the hot products of combustion after the separator extracts coarse ash particulate for producing steam, a steam turbine coupled to a generator for expanding the steam and producing power, and a condenser unit for condensing steam expanded in the steam turbine. A pump may be provided for returning the condensate produced by the condenser to the waste heat boiler.
Separation, and/or precipitation filtering, of much of the ash particulate from the products of combustion before these products are applied to the waste heat boiler significantly reduces fouling of the heat transfer elements in the boiler. Moreover, under these conditions, a spouted bed combustor is advantageous over a fluidized bed combustor in that a spouted bed combustor is less sensitive to variations in flow rate through the gas turbine unit which occur under certain conditions of use.
If little or no water is available, an air-cooled condenser can be used. In such case, the condenser unit of the waste heat boiler preferably contains an organic fluid that is vaporized by the condensing steam for producing vaporized organic fluid, and an organic vapor turbine coupled to a generator is provided to produce power by expanding the vaporized organic fluid. The expanded vaporized organic fluid is condensed to a liquid in the air-cooled condenser; and a pump or gravitational assistance may be provided for returning the condensed liquid to the organic condenser.
When a spouted bed combustor is utilized, the power plant according to the invention preferably includes a pyrolyzer adapted to contain crushed oil shale, or other solid fuel, or mixture of fuels, and to receive ash extracted by the separator for producing a carbonaceous residue that is supplied to the spouted bed combustor, and combustible products. The combustible products in the form of combustible gases may be mechanically filtered and then pressurized before being applied to the burner of the gas turbine unit. Alternatively, the combustible products may be treated to produce clean, treated combustibles gases that are applied to the burner, and liquid fuel that may be added to a combustor like a spouted bed combustor. Finally, when a fluidized bed combustor is employed, an auxiliary heat exchanger associated with the combustor may be utilized for exchanging heat between combustor and air compressed by the compressor upstream of said burner.
If an appropriate fluidized bed combustor is used instead of a spouted bed combustor, the heat exchanger would be mounted within the combustion section. When a spouted bed combustor is used, the heat exchanger is a unit separate from the combustor.